Dynamic memory. EECS 211 Winter 2019

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1 Dynamic memory EECS 211 Winter 2019

2 2 Initial code setup $ cd eecs211 $ curl $URL211/lec/06dynamic.tgz tar zx $ cd 06dynamic

3 3 Oops! I made a mistake. In C, the declaration struct circle read_circle(); means that read_circle takes any number of arguments.

4 3 Oops! I made a mistake. In C, the declaration struct circle read_circle(); means that read_circle takes any number of arguments. In traditional C, arguments weren t checked: double min2(); // declaration double min3() // definition double x, y, z; { return min2(x, min2(y, z));

5 3 Oops! I made a mistake. In C, the declaration struct circle read_circle(); means that read_circle takes any number of arguments. In traditional C, arguments weren t checked: min2(); // declaration min3() // definition int x, y, z; { return min2(x, min2(y, z));

6 3 Oops! I made a mistake. In C, the declaration struct circle read_circle(); means that read_circle takes any number of arguments. In traditional C, arguments weren t checked: min2(); // declaration min3() // definition int x, y, z; { return min2(x, min2(y, z)); The correct way to say no arguments in C is struct circle read_circle(void);

7 And now, strings... 4

8 5 How can we work with strings? bool is_comment(const string*); // Concatenates array of strings; strips comments. string strip_concat(const string* begin, const string* end) { string result = ""; while (begin < end) { if (! is_comment(begin)) result += *begin + "\n"; ++begin; return result;

9 5 How can we work with strings? bool is_comment(const string*); // Concatenates array of strings; strips comments. string strip_concat(const string* begin, const string* end) { string result = ""; while (begin < end) { if (! is_comment(begin)) result += *begin + "\n"; ++begin; return result; This is actually C++.

10 5 How can we work with strings? bool is_comment(const string*); // Concatenates array of strings; strips comments. string strip_concat(const string* begin, const string* end) { string result = ""; while (begin < end) { if (! is_comment(begin)) result += *begin + "\n"; ++begin; return result; This is actually (very inefficient) C++.

11 6 Where should strings live? Solution in each function s automatic storage in one function s automatic storage someplace else

12 6 Where should strings live? Solution in each function s automatic storage in one function s automatic storage someplace else Problem

13 6 Where should strings live? Solution in each function s automatic storage in one function s automatic storage someplace else Problem inflexible & inefficient

14 6 Where should strings live? Solution in each function s automatic storage in one function s automatic storage someplace else Problem inflexible & inefficient functions return

15 6 Where should strings live? Solution in each function s automatic storage in one function s automatic storage someplace else Problem inflexible & inefficient functions return difficult

16 7 A uniform-capacity string Can be passed, returned, assigned: #define MAXSTRLEN 80 struct string80 { char data[maxstrlen + 1]; ; typedef struct string80 string80_t; The easy-but-inflexible solution: all strings have the same capacity See src/string80.h

17 8 So we work with '\0'-terminated char*s The C string: void copy_string_into(char* dst, const char* src) { while ( (*dst++ = *src++) ) { This works provided src is actually terminated and dst has sufficient capacity See str/ptr_string.c

18 8 So we work with '\0'-terminated char*s The C string: void copy_string_into(char* dst, const char* src) { while ( (*dst++ = *src++) ) { This works provided src is actually terminated and dst has sufficient capacity See str/ptr_string.c But how can we ensure that dst has sufficient capacity?

19 9 Okay, but where should we store dst? #include "ptr_string.h" #include <stdio.h> int main() { // Actually stored in the static area : const char message[] = "On the stack!"; // Stored in main s stack frame: char buf[sizeof message]; copy_string_into(buf, message); printf("%s\n", buf); str_toupper_inplace(buf); printf("%s\n", buf);

20 10 This function is wrong, and cannot work #include "ptr_string.h" char* bad_str_toupper_copy(const char* s) { char result[count_chars(s) + 1]; str_toupper_into(result, s); return result; Why?

21 10 This function is wrong, and cannot work #include "ptr_string.h" char* bad_str_toupper_copy(const char* s) { char result[count_chars(s) + 1]; str_toupper_into(result, s); return result; Why? The result points to an object that is destroyed when bad_str_toupper_copy returns.

22 11 Dynamic memory allocation: The basics Function void* malloc(size_t size) requests size bytes of memory from the system.

23 11 Dynamic memory allocation: The basics Function void* malloc(size_t size) requests size bytes of memory from the system. malloc() either returns a pointer to a new object of the requested size, or indicates failure by returning special pointer-to-nowhere NULL. (Type void* literally means pointer to nothing, but better to think of it as a pointer to uninitialized memory of unknown size.)

24 11 Dynamic memory allocation: The basics Function void* malloc(size_t size) requests size bytes of memory from the system. malloc() either returns a pointer to a new object of the requested size, or indicates failure by returning special pointer-to-nowhere NULL. Function void free(void* ptr) releases memory back to the system. (Type void* literally means pointer to nothing, but better to think of it as a pointer to uninitialized memory of unknown size.)

25 12 Dynamic memory allocation: The rules 1. Every pointer returned by malloc() must be NULL-checked (because dereferencing NULL is UB)

26 12 Dynamic memory allocation: The rules 1. Every pointer returned by malloc() must be NULL-checked (because dereferencing NULL is UB) 2. Every object returned by malloc() must have its address passed to free() exactly once (because otherwise you leak memory)

27 12 Dynamic memory allocation: The rules 1. Every pointer returned by malloc() must be NULL-checked (because dereferencing NULL is UB) 2. Every object returned by malloc() must have its address passed to free() exactly once (because otherwise you leak memory) 3. After an object is freed, it must not be accessed (read or written) or freed again (or else UB)

28 12 Dynamic memory allocation: The rules 1. Every pointer returned by malloc() must be NULL-checked (because dereferencing NULL is UB) 2. Every object returned by malloc() must have its address passed to free() exactly once (because otherwise you leak memory) 3. After an object is freed, it must not be accessed (read or written) or freed again (or else UB) 4. A object that was not obtained from malloc() must not be freed (or else nasal demons)

29 12 Dynamic memory allocation: The rules 1. Every pointer returned by malloc() must be NULL-checked (because dereferencing NULL is UB) 2. Every object returned by malloc() must have its address passed to free() exactly once (because otherwise you leak memory) 3. After an object is freed, it must not be accessed (read or written) or freed again (or else UB) 4. A object that was not obtained from malloc() must not be freed (or else nasal demons) 5. Except: free(null) is just fine

30 13 Heap allocation example #include "ptr_string.h" #include <stdlib.h> char* string_clone(const char* s) { char* result = malloc(count_chars(s) + 1); if (result) copy_string_into(result, s); return result; char* str_toupper_clone(const char* s) { char* result = malloc(count_chars(s) + 1); if (result) str_toupper_into(result, s); return result;

31 14 Concatenating two strings, result in the heap #include <stdlib.h> #include <string.h> char* string_concat(const char* s, const char* t) { size_t s_len = strlen(s); // count_chars size_t t_len = strlen(t); char* result = malloc(s_len + t_len + 1); if (result == NULL) return NULL; strcpy(result, s); strcpy(result + s_len, t); // copy_string_into return result;

32 15 Our initial example char* strip_concat(char** lines, size_t count) { size_t total_len = 0; for (size_t i = 0; i < count; ++i) if (! is_comment(lines[i])) total_len += strlen(lines[i]) + 1; char* result = malloc(total_len + 1); if (result == NULL) return NULL; char* fill = result; for (size_t i = 0; i < count; ++i) { if (! is_comment(lines[i])) { fill = stpcpy(fill, lines[i]); *fill++ = '\n'; *fill = '\0'; return result; See src/string_fun.c and test/test_string_fun.c.

33 Next: Linked data structures 16

34 17 NULL versus nul versus null Thing Type of Thing Purpose of Thing

35 17 NULL versus nul versus null Thing Type of Thing Purpose of Thing [a] null [pointer] T* for any T stands for a missing object

36 17 NULL versus nul versus null Thing Type of Thing Purpose of Thing [a] null [pointer] T* for any T stands for a missing object NULL void* null pointer constant

37 17 NULL versus nul versus null Thing Type of Thing Purpose of Thing [a] null [pointer] T* for any T stands for a missing object NULL void* null pointer constant '\0' (a/k/a nul) int 0 with character connotation

38 17 NULL versus nul versus null Thing Type of Thing Purpose of Thing [a] null [pointer] T* for any T stands for a missing object NULL void* null pointer constant '\0' (a/k/a nul) int 0 with character connotation So NULL is null, but nul is something completely different.

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